Thin films of organic semiconductors have been cast into n- and p-type transistors separately, but the world's first gate to use both was recently fabricated at the University of Washington (Seattle) and Stanford University (Palo Alto, Calif.).

The complementary organic semiconductor (COS) circuitry was created from nanowires that self-assembled at room temperature from solutions of organic semiconductors. The semiconductor inverter gate that was produced (hexathiapentacene for p-type and perylenetetracarboxyldiimide for n-type) had a gain exceeding of 8, an on/off ratio of 104 and electron mobility on the order of one-hundredth of a square centimeter per volt-second.

"This is the first all organic nanowire integrated circuit," said graduate researcher Alejandro Briseno at the University of Washington. "Others have demonstrated organic n-type and p-type thin-film transistors fabricated via high-temperature thermal evaporation, but our method not only provides a simple solution-processable method of fabricating inverters, but it also demonstrates the possibility of fabricating single-crystal nanowire transistors from both p- and n-type transistors at room temperature."

Complementary semiconductors are important because they reduce power consumption by using capacitively coupled inputs that consume energy only when they switch from a 1 to a 0 or vice versa. Complementary organic semiconductors use the same energy-efficient complementary architecture as complementary metal oxide semiconductors (CMOS) but are cast in inexpensive organics instead of inorganic metal oxides.

The organic nanowire transistors are called one-dimensional because their channels self-assemble into nanowires so narrow that, mathematically, they can be treated as having only length. Compared with silicon semiconductors, organics ordinarily have much lower gain, on/off ratio and electron mobility, but by going to one-dimensional nanowires, much of that performance loss can be regained, say the researchers.

Because the nanowires' diameters are measured in nanometers and their growth patterns were random in the demonstration chip, arrays of nanowires were grown atop the respective source and drain electrodes to the p- and n-type transistors. First, the nanowires were self-assembled in a solution spread as a thin film, resulting in random nanowire arrays precipitated out of the organic semiconductor solution atop the electrodes.

"The use of organic nanomaterials in a basic complementary inverter has not yet been demonstrated until now," said Briseno. "We were able to accomplish this by synthesizing large quantities of crystalline nanowires from a variety of low-cost, commercially available semiconductors via a solution-phase process."

The electron mobility of the transistors in the organic inverter was measured by the research team as one-hundredth of a square centimeter per volt-second, compared with just under one square meter per volt-second for silicon—a 1,000-fold difference. However, the team has high hopes of improving the electron mobility of organic semiconductor circuitry.

"Now, we need to work on synthesizing new organic materials that are solution-processable and that also have efficient charge transport," said Briseno. "This is a challenge that the entire scientific community is actively pursuing."

If organic semiconductors can be perfected, they promise to be inexpensive enough for practical use in disposable applications such as the fabrication of complex circuitry in radio-frequency identification (RFID) tags.